Parts of Northland had their warmest and wettest winter on record as New Zealand sweltered through its warmest winter ever recorded.
And the extreme winter could become the new norm, Niwa principal scientist – forecasting Chris Brandolino said.
The Seasonal Climate Summary for the three months of June to the end of August released by Niwa shows winter 2020 was New Zealand's warmest winter on record. The nationwide average temperature was 9.6C (1.1C above the 1981-2010 average from Niwa's seven-station temperature series which begins in 1909).
Brandolino said the season was characterised by mean sea level pressures that were higher than normal to the east and southwest of New Zealand, with slightly lower than normal sea level pressure northwest of the country. This pressure pattern delivered more frequent warm northeasterly winds than normal, particularly to the North Island.
He said this was highlighted by the massive rainfall that hit Northland on July 17, causing widespread flooding and slips, with State Highway 1 through the Mangamukas in the Far North still closed due to a large slip.
Rainfall totals in Northland were well above normal (more than 149 per cent) of the winter normal and the July 17 storm saw daily rainfall totals of 262mm at Kaikohe and 251mm at Whangārei.
Brandolino said these are the highest maximum one-day rainfall amounts on record for winter at those locations, contributing 28 per cent and 31 per cent of the total amounts recorded at Kaikohe and Whangārei respectively for winter as a whole.
Kaikohe recorded 935mm of winter rain - its second-highest total for the season since records began in 1956, and Whangārei had 810mm of rain - its fourth-highest winter total since records began there in 1937.
Kerikeri recorded a mean air temperature for the season of 12.9C - a 1.1C increase on the norm, and the highest winter total since records began there in 1945.
Cape Reinga recorded a mean air temperature of 13.5C, its second-highest since
records began there in 1951, the same mean as Kaitaia, where it was the second-highest since records began there in 1948.
Whangārei, meanwhile, had a mean air temperature or 13C, the city's second-highest on record for the season.
Whangārei also recorded its highest ever mean maximum air temperature of 17.2C, a 1.3C increase on the normal. Kerikeri's mean maximum air temperature of 17.2C was its second-highest on record.
Brandolino said climate change, along with some other weather features all contributed to the warm winter, and with climate change only set to intensify, the country could expect more warmer-than-normal winters in the future.
''That's because the Earth is warmer now than it was 50 or 100 years ago and our ocean temperatures this past winter are warmer than normal - in the Northern North Island by 0.67C warmer than normal - which has an influence,'' he said.
Sea surface temperatures (SSTs) surrounding New Zealand were also warmer than average during winter, most notably during August, and this exerted a further warming influence on the country's air temperatures.
''This combination of more frequent northeasterlies, warmer SSTs, and higher pressure over the country, along with a background influence of climate change [ie a long-term increase in average temperatures], resulted in widespread warm conditions during winter,'' he said.
NORTHLAND WINTER WEATHER HIGHLIGHTS:
■ June 26 - three separate small tornadoes were reported in Northland. In Whangārei, six boats were blown off their cradles in the Norsand boatyard. In Mata, trees were toppled and iron from a farm shed was strewn among a nearby stand of trees. Further north in Oakura, trees were also toppled by a tornado.
■ July 17 - very heavy rain and thunderstorms hammered much of Northland. Civil Defence welfare centres were activated as people were forced to leave their homes due to flooding. Approximately 65 homes were evacuated, and four of these homes were left uninhabitable due to damage sustained by floodwaters. Police advised against non-essential travel throughout Northland due to widespread and considerable flooding, particularly around Whangārei. Whangārei Airport recorded 50.8mm of rain between 9-10 pm, the city's second-highest hourly rainfall total for all months on record.
■ July 17 - approximately 500 lightning strikes were recorded near Northland. They were associated with thunderstorms that delivered heavy downpours of rain to the region.
■ August 18 - a period of heavy rain occurred in parts of Northland, with 69 mm recorded at Kerikeri. While the rainfall totals observed were not especially high, they were cause for concern as much of Northland soils were still saturated by heavy rain and flooding that occurred during the previous month.
■ August 20 - a tornado was reported in Pukenui in the Far North. A launch was blown off its blocks and one house lost half of its roof. Additional damage was reported to include trees, a large shed and a trampoline which was sent tumbling along the road.